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The most basic and obvious difference is that analogue signals can be any value within a defined range, whereas digital signals can only take a limited number of values within a defined range. See figure 1.

In fig 1, if you look at the voltage for the analogue signal, you can see that it varies anywhere between the defined voltage range, whereas if you look at the voltage range for the digital signal, it is either on or off, the range is either maximum voltage or none at all.

Not all digital signals are as simple as one shown in Fig 1. There’s what is known as ‘carrier modulation schemes’ that are used to enable more data to be transferred through communications systems. Carrier signals have three physical properties, frequency, amplitude and phase. By manipulating these properties it is possible to send more data. One example of a carrier modulation scheme would be QAM 64.

There are disadvantages to using analogue signalling when transferring data. The main disadvantage to using analogue transmission is that if the signal strength (voltage) is weakened in the transmission in some way, because the voltages do not have to be specific unlike digital, the data sent can be interpreted incorrectly. The signal can be very easily distorted by other electrical signals, which will also cause data to be interpreted incorrectly.

The problem with data transfer is that computers can’t understand analogue signals, computers work with digital signals. Over certain systems, such as ISDN or PTSN, the signal needs to be converted between analogue and digital, as the data signal travels over standard telephone lines. A hardware device is required to convert between the two signals; we call this device a ‘modem’. A modem or modulator-demodulator is the name of the piece of hardware that is used to convert the between the two signals. When transmitting the signal, the modem ‘modulates’ the signal, when receiving it ‘demodulates’ the signal.

There is an organisation known as the “Comité Consultatif International Téléphonique et Télégraphique”, the translation to English means “International Consultative Committee on Telecommunications and Telegraphy”. It is an international organisation, which developed standards of modems to ensure compatibility. Prior to the standards developed by this organisation, companies like the American Telephone and Telegraph Company set their own standards, but were not universally adhered unlike the current V numbers.

Below is a description of what the V classifications are. Although not the most recent in the series, the most common modem in use today is the V.34bis. The first standard to be internationally recognised was the V.22bis

V.22
Provides 1200 bits per second at 600 baud (state changes per second)

V.22bis
The first true world standard, it allows 2400 bits per second at 600 baud

V.32
Provides 4800 and 9600 bits per second at 2400 baud

V.32bis
Provides 14,400 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second

V.32turbo
Provides 19,200 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second; can operate at higher data rates with compression; was not a CCITT/ITU standard

V.34
Provides 28,800 bits per second or fallback to 24,000 and 19,200 bits per second and backwards capability with V.32 and V.32bis

V.34bis
Provides up to 33,600 bits per second or fallback to 31,200 or V.34 transfer rates

V.35
The trunk interface between a network access device and a packet network at data rates greater than 19.2 Kbps. V.35 may use the bandwidths of several telephone circuits as a group. There are V.35 Gender Changers and Adapters.

V.42
Same transfer rate as V.32, V.32bis, and other standards but with better error correction and therefore more reliable

V.90
Provides up to 56,000 bits per second downstream (but in practice somewhat less). Derived from the x2 technology of 3Com (US Robotics) and Rockwell's K56flex technology

Last edited by Konshuss; January 8th, 2007 at 07:41 PM.
Reason: Wasn't sure where the download box was for the word DOC.